A miracle is an exception to the general laws of the universe, arranged by a deity, possibly at the behest of one of its followers. These take two general forms:
Earlier times[]
In ancient writings, and slightly beyond the operating range of working video cameras, miracles involve such matters as golden plates deciphered by the use of scrying stones, prophets brought unto the heavens on flying horses, the dead rising from their graves, and a truly unique solution to a wine shortage at a wedding, much to the relief, one presumes, of the caterer.
Modern times[]
In more modern times, a miracle is such a thing as unidentified oleaginous gunk oozing from the eyes of statues and painted icons while those in charge of those sacred object will not allow close examination. Further examples of miracles can be apparent or reported improvement in diseases and medical conditions with generally subjective assessments of severity and known rates of remission.
You must understand, modern gods simply do not return severed limbs to their attached state. It's just not done anymore, apparently, in divine circles. But if you'd like your tension headache to improve subtly after you took that ibuprofen, they might be able to help out, there. If they're in the mood.
Third category[]
A third category of miracle--the fact that anyone believed a huckster with his head in a hat and a set of scrying stones, much less a Brylcreemed monstrosity waving around a weaponized jacket--has also been mooted for consideration within the general class of phenomena.